1. Field of the Invention
This invention relates to a process of ultrasonic pasteurization wherein, the time and temperature needed to kill non-sporulated bacteria, vegetative cells fungi and viruses, on contaminated surfaces, is reduced.
More specifically, it deals with liquid phase pasteurization in either heated water or aqueous liquids wherein objects to be pasteurized are placed and irradiated with an ultrasonic field below 150 KHz and an average acoustic energy density greater than 5 watts per liter for from 15 to 30 minutes.
2. Description of the Prior Art
In food science, pasteurization is defined as a heat treatment that kills part but not all of the vegetative microorganisms present in the food and, consequently, is used for foods which are further handled and stored under conditions which minimize growth (i.e., refrigeration). However, in some cases (milk, for instance) pasteurization corresponds to the complete destruction of all pathogenic microorganisms. Minimum legal time-temperature relationships have been established for pasteurization of milk. These are (1) the low-temperature, long-time method (Holder process) in which every particle of milk is heated to 62.8.degree. C. and held at that temperature for 30 minutes; and (2) the high-temperture, short-time method (H-T, S-T or "flash" process) involving an exposure of milk to 71.7.degree. C. for 15 sec. Another method, that of ultra-high temperature (UHT) employs temperatures near 93.3.degree. C. or above for a fraction of a second.
In world-wide practice, one or more of four general temperature zones are in use for heat-treating milk. These are (1) 62.8.degree. C. for 30 min. and/or 71.7.degree. C. for 15 sec.; (2) 79.4.degree. C.-90.6.degree. C. for 15 sec. or less; (3) 93.3.degree. C.-100.degree. C., and (4) above 107.2.degree. C. momentarily up to 30 min.
When handling non-food material, the pasteurization method means, in general, the complete destruction of the various non-spore forming pathogens which can be encountered in each particular situation. One, therefore, must accurately define the type of microorganisms which may be encountered in a specific problem before claiming complete destruction of all non-spore formers by pasteurization. For instance, it has been shown that Mycobacterium tuberculosis has a thermal death point of 60.degree. C. for 20 minutes, while Staphylococcus aureus has a thermal death point of 65.6.degree. C. for 30 minutes.
It has recently been established that a satisfactory decontamination of anesthesia, resporatory therapy and urology equipment can be achieved by submerging said equipment into hot water during 30 minutes at a temperature of 76.7.degree. C. (170.degree. F.). This particular pasteurization or hot water disinfection technique is said to be based mainly on the coagulation and denaturization of the microorganism proteins.
Since the early thirties (Schmitt F. O., Uhlenmeyer B., The Mechanism of the Lethal Effect of Ultrasonic Radiation, Proc. Soc. Exptl. Biol. Med., 27:626-628, 1930), the cidal effects of ultrasonics on microorganisms suspended in liquid media has been well-recognized. The ultrasonic approach has been widely used both at laboratory and industrial scale to kill microorganisms, disrupt them or extract valuable compounds (enzymes, etc.). Theory and practice have both shown that lethal effects are a function of cavitation intensity and this is why, today, all large scale industrial processing applications are confined to the frequencies below 150 KHz.
In a cavitating field, microorganisms are submitted to the large amplitude shock waves released after the collapse of vapor filled resonant bubbles. These shock waves disrupt agglomerates and damage the protective membrane of microorganisms. They can physically break large molecules or viruses into smaller entities. Essentially, the same mechanism is used in cleaning applications. In this case, the pressure shock waves clean the surface of a material by jarring or knocking the scale or dirt from the surface. If exposing a metal over extended periods of time in a low frequency cavitating field, "pitting" will be observed on the metal interface. The amount of metal removed by shock wave erosion is often used to measure the intensity of the cavitation. One also should add that other phenomena (production of free radicals, H,OH,HO.sub.2, and toxic agents such as ozone or hydrogen peroxide) take place in a water cavitating field which can also contribute to the death of microorganisms. The denaturization of enzymes (Macleod R. M., Dunn F. J.; Acous. Soc. Am., vol. 42, no. 2:527-529, 1967) such as Trypsid, .alpha.-chymotrypsin, and Lactate dehydrogenase, has also been mentioned recently as an important factor to affect microorganism metabolisms.
It is apparent, however, that any process which will reduce the time necessary to decontaminate instruments or containers is most welcome in industry and any process which will accomplish this at a lower temperature will allow the use of a wider variety of materials from which said instruments or containers can be produced.